For safety reasons, it is important to monitor the speed of a rotating shaft in machinery, such as the rotating shaft in a turbine. For example, if the shaft rotates to quickly during normal operations, then the machinery may become damaged. Additionally, if the shaft rotates too quickly, the machinery may begin to break apart posing a risk to operators and individuals near the machinery. Similarly, low speed conditions may also cause damage to the machinery and may also indicate that operational problems with the machinery exist and need to be addressed. Attempts to monitor for different speed conditions exist, but these systems have typically provided little protection and often have been costly.
For example, some of these prior systems have provided either overspeed or underspeed protection, but not both. Prior systems which have provided underspeed protection have failed to provide a time delay which permits the rotating shaft an opportunity to reach its operational speed without triggering a shutdown and an override if the minimum speed has been reached before the time period has expired. As a result, these systems have permitted machines to operate at unsafe rotational speeds for the shafts.
Another problem with some prior systems is with determining when to shut down the machinery if an overspeed condition exists. These systems rely upon a two-out-of-three voting arrangements to shut the machinery during for an overspeed condition. Although this arrangement is less likely to trigger false or unnecessary shutdowns, it is also more likely to permit the machinery to continue to operate when a shutdown is necessary. Additionally, if one of the set of three components fails, then the system will only shut down the machinery if the other two sets of components signal the need for a shutdown. If two sets of components fail, then these prior system will not work at all. As a result the machinery is more likely going to be permitted to run at unsafe speeds. Further, two out of three voting systems are expensive because they require three different sets of components to operate.